New face affects close by faces
What you are seeing is Blender's smooth shading view trying to smooth out the appearance of the faces. Whenever you add new faces, you give Blender more faces to try and shade smooth. Before adding the face, the original face was an edge. Now you are telling Blender that it is actually a corner, and thus the shading is updated. Now, the other reason for how the shading looks is that the original face is an ngon, so it has more than 4 faces. Ngons are much more prone to shading issues, as they have to be triangulated before rendering, and this can lead to unexpected behavior. Especially in this case, you should try to avoid them. Look into trying to clean that up and make your topology better. There are plenty of resources out there for that, both on this site and elsewhere.